A. Field of the Invention
The present invention relates to a device for cool drying gas.
B. Related Art
More specifically, the invention relates to a device that consists of a heat exchanger whose primary part is the evaporator of a cooling circuit and a gas to be dried flows through the secondary part thereof, which cooling circuit is filled with a coolant and further contains a compressor and a condenser and first expansion means between the condenser outlet and the inlet to the above-mentioned evaporator, whereby there is at least one bypass pipe that connects the discharge side of the compressor to the intake side of the compressor, whereby second expansion means are provided in this bypass pipe and a control valve which is controlled by a control unit as a function of signals received from one or more measuring elements.
As is known, cool drying is based on the principle that by reducing the gas temperature, the moisture in the gas condenses, after which the condensed water is separated in a liquid separator and after which the gas is again heated such that it is no longer saturated.
Compressed air which is for example delivered by a compressor is generally saturated with water vapour, or in other words has a relative humidity of 100%. This means that condensation occurs in the event of a temperature drop to below the so-called dew point. As a result of the condensed water, corrosion will occur in the piping and tools and the equipment can exhibit premature wear.
Hence compressed air is dried, which can be done in the above-mentioned way by cool drying. Air other than compressed air or other gases can also be dried in this way.
When drying compressed air, the air in the heat exchanger must not be cooled down too much as the condensate would then freeze. Typically the dried compressed air has a temperature of two to three degrees above zero or 20 degrees Celsius below ambient temperature. The temperature of the coolant in the evaporator is kept between 15° C. and −5° C. for this purpose.
Traditionally the temperature of the coolant is kept under control by equipping the device with at least one bypass pipe over the compressor. A control valve in an above-mentioned bypass pipe enables a certain (extra) quantity of coolant to be branched off from the cooling circuit to then be led through the above-mentioned bypass pipe over the compressor.
The control valve is hereby controlled by a control unit, also called a controller, which is connected in a known way to one or more measuring elements.
A device is already known in which one or more measuring elements are provided on the outside of the heat exchanger to measure the lowest gas temperature, also called the LAT. The LAT is the lowest occurring temperature of the gas to be dried that it is guided through the secondary part of the above-mentioned heat exchanger. Consequently, the LAT has to be measured by measuring elements which are placed where the temperature is expected to be equal to that of the gas when the cool drying device is in operation.
When the above-mentioned measuring elements register a lowest gas temperature (LAT) whereby freezing of the condensate can occur, the control unit sends a signal to the control valve to open it. In this way a certain quantity of coolant or an extra quantity of coolant is led over the compressor via an above-mentioned bypass pipe such that the cooling capacity of the cooling circuit decreases.
If the lowest gas temperature (LAT) is more than two to three degrees above zero, the control valve is closed such that the entire capacity of the cooling circuit is used to sufficiently cool the gas to be dried.
Tests have shown that it is not easy to position the measuring elements on the heat exchanger to measure the lowest gas temperature (LAT). Moreover, with some heat exchangers it has turned out to be impossible to measure the lowest gas temperature (LAT) in the above-mentioned way. It is clear that in such cases the control valve cannot be controlled on the basis of a temperature that corresponds to the lowest gas temperature (LAT).
Another disadvantage is that there is a certain time delay between the measured temperature and the actual presence of the lowest gas temperature (LAT) in the secondary part of heat exchanger at a certain point in time. This implies that the control is also executed with a certain time delay.
A further disadvantage is that, at both a high and low load of the device, the measured lowest gas temperature (LAT) deviates from the dew point of the gas, such that freezing of the condensate can nevertheless occur.
WO 2007/022604 describes a device that is equipped with a control unit to which a first measuring element is connected to measure the coolant temperature and a second measuring element to measure the lowest gas temperature (LAT) or dew point.
In one operating mode of this device, the cooling circuit is switched on and off on the basis of the measurement of the lowest gas temperature. In the event of zero load or of a small partial load, the cooling circuit is switched off by means of a third measuring element in the form of a flow sensor.
The measured value of the coolant temperature is used to control the control valve in the bypass pipe of the cooling circuit.
A disadvantage of such a device according to WO 2007/022604 consists of the use of a flow sensor such that the cost increases and the device is rather more prone to failure, and in particular depends on the good operation of the above-mentioned flow sensor to be able to adapt itself to full load or zero load conditions.
A further disadvantage of such a device consists in that the measuring element to measure the lowest gas temperature (LAT) is also in this case placed outside of the heat exchanger.
EP 1.103.296 describes a device that is equipped with two measuring elements to measure the evaporator temperature and evaporator pressure, which measuring elements are connected to a control unit to control the rotational speed of the compressor in the cooling circuit and to control the control valve in the bypass pipe. No account is taken of the lowest gas temperature (LAT).
According to another embodiment of the device according to EP 1.103.296 the above-mentioned measuring elements to measure the evaporator temperature and the evaporator pressure can be replaced by a measuring element for determining the lowest gas temperature (LAT), but in such a case no account is taken of the temperature of the coolant in the cooling circuit.
The disadvantages of such embodiments that only control the control valve on the basis of a signal from a measuring element that measures the lowest gas temperature (LAT) have already been discussed above. If the control is only based on the measured temperature of the coolant, the control will not be optimal, as the measured temperature value does then not correspond with the dew point.